EP2595352B1 - Verfahren und Vorrichtung zur verbesserten Kanalschätzung unter Verwendung einer frame-basierten Kompensation der Kanaländerungen - Google Patents

Verfahren und Vorrichtung zur verbesserten Kanalschätzung unter Verwendung einer frame-basierten Kompensation der Kanaländerungen Download PDF

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EP2595352B1
EP2595352B1 EP11189856.5A EP11189856A EP2595352B1 EP 2595352 B1 EP2595352 B1 EP 2595352B1 EP 11189856 A EP11189856 A EP 11189856A EP 2595352 B1 EP2595352 B1 EP 2595352B1
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sub
carrier
ofdm
channel estimate
change
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EP2595352A1 (de
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Andrei Popescu
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Qualcomm Technologies International Ltd
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Cambridge Silicon Radio Ltd
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Priority to CN201210019367.9A priority patent/CN103124243B/zh
Priority to US13/679,942 priority patent/US8787504B2/en
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/022—Channel estimation of frequency response
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/0224—Channel estimation using sounding signals
    • H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only

Definitions

  • the invention relates to the field of digital data communications.
  • Receivers in packetized radio communication systems using OFDM modulation require a channel estimate in order to perform reception.
  • the standard way to compute the channel estimate used for reception of an 802.11n OFDM frame is by using information contained solely within that frame, primarily training fields ('long training fields' or 'LTFs' in 802.1 In terminology) that are part of the frame preamble.
  • receiver sensitivity will be limited by the inaccuracy of a channel estimate computed solely based on the LTFs and data symbols of a noisy frame.
  • Known techniques that improve the channel estimate include channel tracking and channel estimate smoothing. However, use of these techniques is not sufficient to render negligible the loss in receiver sensitivity due to channel estimate inaccuracy.
  • the channel estimate is generally computed as a linear combination of the values L m ( k ) for different m.
  • a simple case is that of a single-antenna receiver receiving frames with single space-time-stream modulation, where only one LTF is used to compute the channel estimate.
  • the LTF signals observed by a receiver in consecutive frames are different even if the channel is the same. A number of factors characteristic of typical receiver and transmitter circuits may contribute to this.
  • the difference between the RF carrier angle of the receiver and transmitter changes over time.
  • the LTF is sampled at a different time in different frames, because there is a difference between the baseband sampling clocks at the transmitter and receiver, and/or the frame (boundary) timing estimation is different in different frames.
  • the receiver performs automatic gain control and selects different gains for receiving different frames (and also the gain applied by the transmitter may vary).
  • 802.11n OFDM receivers use the LTFs of a frame to form a channel estimate for processing that same frame.
  • Information from data symbols throughout the frame may however be used to improve the channel estimate.
  • channel tracking can gradually improve the channel estimate as the receiver progresses through processing the frame.
  • the quality of the channel estimate during the first data symbols of a frame is predominantly determined by the quality ('noisiness') of the LTFs of that frame.
  • Channel smoothing is another well known technique that improves the quality of the channel estimate when the received signal is noisy.
  • channel smoothing may not be possible in combination with transmit beamforming. Accordingly, the 802.11n standard has provisions to prevent a receiver from performing channel smoothing.
  • WO 2011/106626 describes a wireless transmit/receive unit that may receive an asymmetrically clipped optical orthogonal frequency-division multiplexing signal.
  • the invention may thus provide means to improve the accuracy of the channel estimate used to receive an OFDM frame (such as a WLAN frame conforming to the IEEE 802.11n-2009 standard), thereby improving the receiver sensitivity.
  • an OFDM frame such as a WLAN frame conforming to the IEEE 802.11n-2009 standard
  • the invention may thus improve the channel estimate used by a packetized system receiver to receive a frame, by using the channel estimate(s) computed from one (or more) previous frame(s).
  • the invention makes use of the fact that consecutive packets transmitted between a transmitter and a receiver in a packetized system may pass through similar instances of the radio channel. This is the case for consecutive packets received from the same transmitter, within a time that is a small fraction of the channel coherence time.
  • the channel coherence time for an 802.11 wireless LAN link is of several hundreds of milliseconds in typical operating environments, whereas the typical duration of data frames is of the order of hundreds of microseconds, up to a few milliseconds.
  • a typical calculation of the frequency-domain channel estimate includes computing an FFT of the received LTF (or possibly of multiple LTFs for MIMO transmissions):
  • the channel estimate is generally computed as a linear combination of the values L m (k) for different m.
  • L m (k) the channel estimate is generally computed as a linear combination of the values L m (k) for different m.
  • a simple case is that of a single-antenna receiver receiving frames with single space-time-stream modulation, where only one LTF is used to compute the channel estimate.
  • the LTF signals observed by a receiver in consecutive frames are different even if the channel is the same. A number of factors characteristic of typical receiver and transmitter circuits may contribute to this.
  • the difference between the RF carrier angle of the receiver and transmitter changes over time.
  • the LTF is sampled at a different time in different frames, because there is a difference between the baseband sampling clocks at the transmitter and receiver, and/or the frame (boundary) timing estimation is different in different frames.
  • the receiver performs automatic gain control and selects different gains for receiving different frames (and also the gain applied by the transmitter may vary).
  • T 0 is the difference between the sampling times of the LTF in the two frames
  • ⁇ 0 is the difference between the carrier angles seen by the receiver in the two frames
  • G 0 is the ratio of the receiver gains used during reception of the two frames.
  • C ( k ) which may be a matrix in a MIMO system, is computed as a linear combination of the values L m (k), it is also related to the previous channel estimate C '( k ):
  • C k C ⁇ k ⁇ e j ⁇ ⁇ 0 ⁇ e j ⁇ 2 ⁇ ⁇ ⁇ k ⁇ T 0 ⁇ G 0
  • channel estimates from different frames can be compared and used in combination.
  • the following scheme based on a parametric model of the ratio between the channel estimates observed by the receiver in different frames, as expected from equation (3), can be used.
  • the channel estimate C(k) is a scalar:
  • the parameter G may be estimated as follows:
  • the parameter T may be estimated as follows:
  • the parameter ⁇ may be estimated as follows:
  • the values G, T and ⁇ can be thought of as representing a shift in the operating state of the system represented by the receiver and the transmitter jointly, occurring over the interval between the frames to which C ( k ) and C' ( k ) relate.
  • G, T and ⁇ can be thought of as representing a shift in the operating state of the system represented by the receiver and the transmitter jointly, occurring over the interval between the frames to which C ( k ) and C' ( k ) relate.
  • ⁇ 0 , To and Go are significant in a given scenario, in which case only the significant ones of ⁇ , T and G need to be estimated.
  • ⁇ and T need to be estimated.
  • the receiver adopts an average of the current and previous channel estimates, in replacement of the current channel estimate:
  • C avg k ⁇ ⁇ C ⁇ align k + 1 - ⁇ ⁇ C k
  • Figures 1 to 3 illustrate the simulated improvement in receiver sensitivity obtained in a static channel without fading, i.e. when the receive signal is only corrupted by additive white Gaussian noise.
  • 'CTF' stands for 'Channel Transfer Function'
  • 'CTF estimation' is what is called ⁇ channel estimation' elsewhere in this document. Note that the improvement in receiver sensitivity attributable to channel estimate averaging is greater for reception of High Throughput Mixed Format frames. This is because the LTF is shorter in these frames (4 ⁇ s) compared with Greenfield and 802.11a 'Legacy' frames where the longer (8 ⁇ s) LTF yields a more accurate channel estimate prior to averaging.
  • the LTFs used in transmissions according to the IEEE 802.11ac draft standard are also 4 us long, hence the improvement in receiver sensitivity attributable to channel estimate averaging when receiving 802.11ac frames is expected to be similar to that obtained when receiving 802.11n High Throughput Mixed Format frames.
  • Figure 4 shows a basic outline of exemplary apparatus for implementing the channel estimation refinement techniques described above.
  • Figure 4 shows an OFDM receiver 10.
  • the receiver 10 comprises an antenna 12, an RF front end 14 and a baseband processor 16.
  • the OFDM receiver 10 will comprise many other features, although they are not shown here since they are peripheral to the theme of the present invention.
  • the OFDM receiver 10 receives radio signals via the antenna 12. These radio signals are demodulated to baseband by the RF front end 14. The baseband signals thus produced are processed by the baseband processor 16. The baseband processor 16 will attempt to demodulate OFDM data from the baseband signals and in doing so the baseband processor 16 employs the channel estimation refinement techniques discussed at length above.

Claims (8)

  1. Verfahren zum Verfeinern einer Kanalzustandsschätzung für einen Hilfsträger in einem orthogonalen Frequenzmultiplexverfahren, wobei das Verfahren umfasst:
    Berechnen einer ersten Kanalzustandsschätzung für den OFDM-Hilfsträger für einen ersten OFDM-Rahmen, der von einem Sender an einem Empfänger empfangen wird;
    Berechnen einer zweiten Kanalzustandsschätzung für den OFDM-Hilfsträger für einen zweiten OFDM-Rahmen, welcher von Sender an dem Empfänger zu einem späteren Zeitpunkt als der erste OFDM-Rahmen empfangen wird;
    Anpassen der ersten Kanalzustandsschätzung zur Minimierung einer Differenz zwischen der ersten und zweiten Kanalzustandsschätzung, um eine Verschiebung in einem Betriebszustand des Senders und des Empfängers zwischen den Empfangszeiten des ersten und zweiten OFDM-Rahmens zu kompensieren, wobei die Verschiebung in dem Betriebszustand eine Veränderung einer Differenz zwischen einem RF-Trägerwinkel des Empfängers und einem RF-Trägerwinkel des Senders und/oder eine Veränderung in der Differenz zwischen den Stichprobenzeitpunkten des Empfängers und Senders und/oder eine Veränderung in einem Verstärkungswert, welcher durch den Sender und/oder Empfänger auferlegt wird, umfasst; wobei das Verfahren gekennzeichnet ist durch:
    Messen einer Diskrepanz zwischen der zweiten Kanalzustandsschätzung und der angepassten ersten Kanalzustandsschätzung;
    Abwandeln der zweiten Kanalzustandsschätzung durch Bestimmen eines Durchschnittswerts der zweiten Kanalzustandsschätzung und der angepassten ersten Kanalzustandsschätzung, wenn die Diskrepanz kleiner als ein vorherbestimmter Wert ist; und
    Demodulieren der Daten von dem zweiten OFDM-Rahmen unter Verwendung der abgewandelten zweiten Kanalzustandsschätzung.
  2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass die Verschiebung in dem Betriebszustand die Veränderung in der Differenz zwischen dem RF-Trägerwinkel des Empfängers und dem RF-Trägerwinkel des Senders umfasst, und der Anpassungsschritt ein Abschätzen der Veränderung umfasst und das Abschätzen der Veränderung umfasst:
    a) Für jeden einer Mehrzahl an OFDM-Hilfsträgern:
    i) Bereitstellen einer ersten Kanalzustandsschätzung dieses Hilfsträgers für den ersten OFDM-Rahmen;
    ii) Bereitstellen einer zweiten Kanalzustandsschätzung dieses Hilfsträgers für den zweiten OFDM-Rahmen; und
    iii) Berechnen des Verhältnisses der ersten und zweiten Kanalzustandsschätzungen für diesen Hilfsträger; und
    b) Ableiten einer Abschätzung der Veränderung von einer Durchschnittsrechnung basierend auf den für die Mehrzahl an OFDM-Hilfsträgern berechneten Verhältnissen.
  3. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass die Verschiebung in dem Betriebszustand die Veränderung in der Differenz zwischen den Stichprobenzeitpunkten des Empfängers und Senders umfasst, und der Anpassungsschritt ein Abschätzen der Veränderung umfasst und das Abschätzen dieser Veränderung umfasst:
    a) Für jeden einer Mehrzahl an OFDM-Hilfsträgern:
    i) Bereitstellen einer ersten Kanalzustandsschätzung dieses Hilfsträgers für den ersten OFDM-Rahmen;
    ii) Bereitstellen einer zweiten Kanalzustandsschätzung dieses Hilfsträgers für den zweiten OFDM-Rahmen; und
    iii) Berechnen des Verhältnisses der ersten und zweiten Kanalzustandsschätzungen für diesen Hilfsträger; und
    b) Ableiten einer Abschätzung der Veränderung von einer Durchschnittsrechnung basierend auf den für die Mehrzahl an OFDM-Hilfsträgern berechneten Verhältnissen.
  4. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass die Verschiebung in dem Betriebszustand die Veränderung in dem Verstärkungswert umfasst, welcher von dem Empfänger und/oder Sender auferlegt wird, und der Anpassungsschritt ein Abschätzen der Veränderung umfasst und das Abschätzen der Veränderung umfasst:
    a) Für jeden einer Mehrzahl an OFDM-Hilfsträgern:
    i) Bereitstellen einer ersten Kanalzustandsschätzung dieses Hilfsträgers für den ersten OFDM-Rahmen;
    ii) Bereitstellen einer zweiten Kanalzustandsschätzung dieses Hilfsträgers für den zweiten OFDM-Rahmen; und
    iii) Berechnen des Verhältnisses der ersten und zweiten Kanalzustandsschätzungen für diesen Hilfsträger; und
    b) Ableiten einer Abschätzung der Veränderung von einer Durchschnittsrechnung basierend auf den für die Mehrzahl an OFDM-Hilfsträgern berechneten Verhältnissen.
  5. Vorrichtung zum Verfeinern einer Kanalzustandsschätzung für einen Hilfsträger in einem orthogonalen Frequenzmultiplexverfahren, wobei die Vorrichtung umfasst:
    Mittel zum Berechnen einer ersten Kanalzustandsschätzung für den OFDM-Hilfsträger für einen ersten OFDM-Rahmen, welcher von einem Sender an einem Empfänger empfangen wird;
    Mittel zum Berechnen einer zweiten Kanalzustandsschätzung für den OFDM-Hilfsträger für einen zweiten OFDM-Rahmen, welcher von dem Sender an dem Empfänger zu einem späteren Zeitpunkt als der erste OFDM-Rahmen empfangen wird;
    Mittel zum Anpassen der ersten Kanalzustandsschätzung zur Minimierung einer Differenz zwischen den ersten und zweiten Kanalzustandsschätzungen, um eine Verschiebung in einem Betriebszustand des Empfängers und des Senders zwischen den Empfangszeiten des ersten und zweiten OFDM-Rahmens zu kompensieren, wobei die Verschiebung in dem Betriebszustand eine Veränderung in einer Differenz zwischen einem RF-Trägerwinkel des Empfängers und einem RF-Trägerwinkel des Senders und/oder eine Veränderung in der Differenz zwischen den Stichprobenzeitpunkten des Senders und des Empfängers und/oder eine Veränderung in einem Verstärkungswert, welcher von Empfänger und/ oder Sender auferlegt wird, umfasst, wobei die Vorrichtung gekennzeichnet ist durch:
    Mittel zum Messen einer Diskrepanz zwischen der zweiten Kanalzustandsschätzung und der angepassten ersten Kanalzustandsschätzung;
    Mittel zum Abwandeln der zweiten Kanalzustandsschätzung durch Bestimmen eines Durchschnittswerts der zweiten Kanalzustandsschätzung und der angepassten ersten Kanalzustandsschätzung, wenn die Diskrepanz kleiner als ein vorherbestimmter Wert ist; und
    Mittel zum Demodulieren von Daten von dem zweiten OFDM-Rahmen unter Verwendung der abgewandelten zweiten Kanalzustandsschätzung.
  6. Vorrichtung nach Anspruch 5, wobei die Verschiebung in dem Betriebszustand die Veränderung der Differenz zwischen dem RF-Trägerwinkel des Empfängers und dem RF-Trägerwinkel des Senders umfasst, und das Anpassungsmittel dazu ausgelegt ist, die Veränderung abzuschätzen und dazu angeordnet ist:
    a) für jeden einer Mehrzahl an OFDM-Hilfsträgern:
    i) eine erste Kanalzustandsschätzung dieses Hilfsträgers für den ersten OFDM-Rahmen bereitzustellen, und
    ii) eine zweite Kanalzustandsschätzung dieses Hilfsträgers für den zweiten OFDM-Rahmen bereitzustellen, und
    iii) das Verhältnis der ersten und zweiten Kanalzustandsschätzungen für diesen Hilfsträger zu berechnen; und
    b) eine Abschätzung der Veränderung von einer Durchschnittsrechnung basierend auf den für die Mehrzahl an OFDM-Hilfsträgern berechneten Verhältnissen abzuleiten.
  7. Vorrichtung nach Anspruch 5, wobei die Verschiebung in dem Betriebszustand die Veränderung der Differenz zwischen den Stichprobenzeitpunkten des Senders und Empfängers umfasst, und das Anpassungsmittel dazu ausgelegt ist, die Veränderung abzuschätzen und dazu angeordnet ist:
    a) für jeden einer Mehrzahl an OFDM-Hilfsträgern:
    i) eine erste Kanalzustandsschätzung dieses Hilfsträgers für den ersten OFDM-Rahmen bereitzustellen, und
    ii) eine zweite Kanalzustandsschätzung dieses Hilfsträgers für den zweiten OFDM-Rahmen bereitzustellen, und
    iii) das Verhältnis der ersten und zweiten Kanalzustandsschätzungen für diesen OFDM-Hilfsträger zu berechnen; und
    b) eine Abschätzung der Veränderung von einer Durchschnittsrechnung basierend auf den Verhältnissen, welche für die Mehrzahl an OFDM-Hilfsträgern berechnet wurden, abzuleiten.
  8. Vorrichtung nach Anspruch 5, wobei die Verschiebung im Betriebszustand die Veränderung des Verstärkungswerts, welcher von dem Sender und/oder Empfänger auferlegt wird, umfasst, und das Anpassungsmittel dazu ausgelegt ist, die Veränderung abzuschätzen und dazu angeordnet ist:
    a) für jeden einer Mehrzahl an OFDM-Hilfsträgern:
    i) eine erste Kanalzustandsschätzung dieses Hilfsträgers für den ersten OFDM-Rahmen bereitzustellen, und
    ii) eine zweite Kanalzustandsschätzung dieses Hilfsträgers für den zweiten OFDM-Rahmen bereitzustellen, und
    iii) das Verhältnis der ersten und zweiten Kanalzustandsschätzungen für diesen Hilfsträger zu berechnen; und
    b) eine Abschätzung der Veränderung von einer Durchschnittsrechnung basierend auf den für die Mehrzahl an OFDM-Hilfsträgern berechneten Verhältnissen abzuleiten.
EP11189856.5A 2011-11-18 2011-11-18 Verfahren und Vorrichtung zur verbesserten Kanalschätzung unter Verwendung einer frame-basierten Kompensation der Kanaländerungen Not-in-force EP2595352B1 (de)

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EP11189856.5A EP2595352B1 (de) 2011-11-18 2011-11-18 Verfahren und Vorrichtung zur verbesserten Kanalschätzung unter Verwendung einer frame-basierten Kompensation der Kanaländerungen
CN201210019367.9A CN103124243B (zh) 2011-11-18 2012-01-20 在数字通信接收机中进行的增强信道估计的方法
US13/679,942 US8787504B2 (en) 2011-11-18 2012-11-16 Enhancing channel estimates made in digital communications receivers

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US9621389B2 (en) * 2013-09-30 2017-04-11 Volvo Car Corporation Method to introduce complementing training symbols into a 802.11p OFDM frame in vehicular communications

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US6856610B2 (en) * 2000-02-28 2005-02-15 Texas Instruments Incorporated Wireless code division multiple access communications system with channel estimation using fingers with sub-chip spacing
US7609786B2 (en) * 2004-01-28 2009-10-27 Qualcomm Incorporated Channel estimation for a communication system using spectral estimation
US8428001B2 (en) * 2005-03-10 2013-04-23 Qualcomm Incorporated Timing corrections in a multi carrier system and propagation to a channel estimation time filter
US8165167B2 (en) * 2005-03-10 2012-04-24 Qualcomm Incorporated Time tracking for a communication system
KR100918500B1 (ko) * 2007-12-03 2009-09-24 한국전자통신연구원 무선 통신 시스템에서의 채널 추정 방법 및 장치
CN101924723B (zh) * 2009-06-09 2013-05-08 中兴通讯股份有限公司 Ofdm信号解调方法和装置
US8737191B2 (en) * 2010-02-25 2014-05-27 Interdigital Patent Holdings, Inc. Blind timing synchronization and low complexity channel estimation in ACO-OFDM systems

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